I too run a bit, mostly in the off season - November - February. Usually 2-3 times a week for about 45 minutes. Does seem to help work joints and tendons and really balances the muscles out.
Lots of time on the rollers when the days are extra nasty. I don't mind riding outside in any condition, but with 4 kids, and working 7 days, sometimes 45 minutes on the rollers is all I can get.
High cadence as well! Seems to keep my legs fresh, but translates to lighter climbing. As light as I can be at least!
For me, it's always been volume of numbers of rides - about 650/year since I commute to work nearly every day. I'm not able to get in huge mileage too many days, but the continuous repetition seems to work and will hopefully pay off.

[Finishing]

My best advice regarding training is that you don't need a plan or a "program".  The tips you've found so far are golden.  How you actually organize or what you actually work on is very personal.  What are your weaknesses?

One example I used to find weaknesses: I had a 200 mile route in a national forest I wanted to do as an overnight.  I picked a number of hours I wanted to hit car-to-car, hit the road one weekend, and did it.  I made my target, found out what I was still bad at.  If I hadn't made that target, I would've had a lot more work to do, but either way I knew what hurt, where I was slow, etc.

Finishing ultra-endurance events requires a base level of fitness and a thick head.  Winning them requires a bit more fitness, but mainly it requires avoiding mistakes:  too much time off the bike, or navigational mistakes.  If you want to do well, you need to practice sleep deprivation and eating in the aerobars.

A few more specifics:

If you can do 6 reps of 300w x 3 minutes (with what recovery period?) routinely, my guess is your power is already in pretty good shape, although it depends on what you weigh.

You'll lose muscle mass.  Any extra muscle mass you can gain anywhere, but especially on your upper body, is a good investment.  Pull-ups, presses, bench presses, kettlebell exercises.

Have a fitness plan for when you finish.  You will feel so weird and depleted, it is very easy to come back, get on the couch, and gain serious pounds.  Since you've already lost muscle, you'll end up a bit of a flabby mess.

I would echo Matt's advice for sure. I was a rookie going into last year, and still consider myself a rookie on the amount of knowledge I am yet to gain.
I rode. As much as I could. Had some goals and baselines for weekly mileage, but I don't use a structured plan. Didn't ignore the early season races that I enjoy doing...racing makes training fun! But tried to ride home from them and rode tired a lot. Sleep deprivation caught up to me a couple times on the divide, but riding tired during the spring paid off.
Being a bigger guy 6'3" 210 (and not really training for anything specific ever) I did do a decent amount of push-ups. They work a big part of your core, back (upper and lower) shoulders, and anything that will fatigue from doing things you're not used to doing; dragging, pushing, climbing, etc. I spent the entire month of may eating, riding, eating.
I'm a flat lander as well, so from day 1 when those climbs hit, I had to use my extra strength to keep up.
I stretched most nights, even just for a minute, before climbing into my bivy.
Also...have that plan for after you finish. Whether racing or just riding it, your body will take a major hit. Relax, recover, but try to stay active. I came home at 197, and by September 1 I was back up to 212. When I hit 218, the most I've ever weighed, a week ago (damn turkey) I figure it's time to start thinking about next year.
For now, I'm going to ride the rollers and finish off this hot chocolate.

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